In the “power electronics” domain, there are now power sources that can deliver DC output voltages which include inverters operating on a “soft-switching” principle.
The electrical circuit diagram of such a known source with a quasi-resonant soft-switching type inverter is represented for reference in FIG. 1 in a functional configuration.
The inverter 2 is connected between a reference terminal 4 and a power supply terminal 6 of a DC voltage source 8.
Switching cells or legs, denoted in general by the reference number 10 and in particular by the references 101 and 102, are arranged in parallel between the terminals 4 and 6 of the voltage source 8. These legs 10 each comprise two switches linked in series between the terminals 4 and 6. The switches are denoted in general by the numeric reference 12 and in particular by the references 121,1, 121,2, 122,1 and 122,2.
Each switch 12 conventionally comprises one or more controllable transistors (MOSFET or IGBT for example), at the terminals of which diodes are mounted in anti-parallel fashion.
Furthermore, each switch 12 of each switching leg 10 is also mounted in parallel with a switching-assisting capacitive element, denoted in general by the numeric reference 14 and in particular by the numeric references 141,1, 141,2, 142,1 and 142,2.
The switching legs 101 and 102 thus each present an output terminal 161 and 162 taken between the two central switches of each leg.
Moreover, each switch 12 of each leg 10 is linked for its control to a control device 16 external to the inverter 2.
The inverter 2 also comprises a transformer 20, the primary of which is linked in series between the two output terminals 161 and 162 of the switching cells 101 and 102.
Furthermore, an inductive element 22 is linked in series between the primary of the transformer and the output terminal 161 of the cell 101 to form a resonant element.
The secondary of the transformer 20 is in turn linked to a rectifier 24, the output terminals 26 of which form the output of the power source, or, in the context of a welding set, the welding terminals.
The operation and control of such a circuit are known in the state of the art.
The control device 18 delivers only turn-off commands to the various switches 12. The switching from an off state to an on state is achieved spontaneously at zero voltage according to the quasi-resonant soft-switching principle, on receipt of a turn-on command sent by the control device 18.
In practice, the reactive energy stored in the resonance elements, or the capacitive elements 14 and the inductive element 22, to which can be added, if appropriate, the spurious capacitances of the switches 12 and the leakage inductance of the transformer 20, is used to obtain spontaneously, at the output terminals 16 of the legs 20, conditions for switching from the off state to the on state corresponding to a soft switching action.
However, conventionally in such a circuit, the commands to turn off and turn on a switch must be separated in time by a dead time, to ensure that the capacitors are fully discharged so as not to allow a switch for which the voltage at the terminals is not zero to be turned on.
For this, the turn-off and turn-on commands must be perfectly synchronized to prevent any short-circuiting of a switching leg.
This raises major problems in welding sets where the current employed is high, such that, to avoid being exposed to a short circuit, it is best to define long dead times.
The problem that is then raised is how to propose an improved arc welding set comprising a power source and a quasi-resonant soft-switching type inverter, which provides a high level of protection against short circuits.